The present invention relates to an electrode substrate used in an electronic device such as a liquid crystal electro-optical device, a method for coating transparent electrodes on such an electrode substrate with a conductive substance, and a mounting method of an IC chip and a film substrate.
Conventionally, since an ITO pattern for external connection that was formed on a glass substrate of a liquid crystal electro-optical device had a high resistance, it was designed to be wide, for instance. When necessary, electroless nickel plating etc. was performed to reduce the electrode resistance. In a method of printing a conductive paste, the conductor width could not be reduced to less than about 100 .mu.m, which, for instance, causes limitations on the pattern shape. This method could not be applied to, for instance, a part of a product having a complex shape. Further, there was a limitation in terms of a manufacturing process, such as a requirement that printing should be performed on a substrate that was in a board-like form.
Conventionally, face-down mounting of an IC chip was performed in the following manner in the case of a liquid crystal electro-optical device, for instance. Solder bumps were formed on an IC chip. A substrate-side pattern of a liquid crystal panel is formed by subjecting an ITO pattern to nickel electroless plating. After the pads of the IC chip and the substrate-side pads of the liquid crystal panel are positioned with respect to each other, soldering is effected by heat-melting the solder bumps of the IC chip with infrared light, etc. Alternatively, as shown in FIG. 13, an anisotropic conductive film 18 of a thermosetting adhesive or a thermosetting and thermoplastic adhesive containing connecting particles that have been produced by subjecting plastic particles to nickel and gold plating and then applying an insulating film thereto are temporarily attached to a substrate. After an IC chip 10 having gold bumps 11 is positioned with respect to ITO electrodes 2 of the substrate, the anisotropic conductive film of the adhesive is set by thermo-compression bonding, and the gold bumps of the IC chip and the ITO electrode of the substrate are connected to each other by breaking the insulating films by applying pressure to and heating the conducting particles interposed therebetween. As a further alternative, gold bumps of about 60 .mu.m in height are formed on an IC chip and an Ag paste is contact-transferred to only the bump portions. After the bumps are positioned with respect to ITO electrodes, connection is made by setting the Ag paste adhesive by heating.
In the case of film substrates such as a TCP and FPC, terminals of a liquid crystal panel and terminals of a film substrate were connected to each other by heating and pressure application using an anisotropic conductive film equivalent to the above-described one or produced by plating plastic particles only with nickel and gold.
In the conventional techniques, in the case of a liquid crystal electro-optical device, the resistance of transparent electrodes cannot be reduced to less than 10 .OMEGA./.quadrature. in the mass-production level. The line width should be properly adapted, because display nonuniformity occurs due to a difference in line resistance if connecting lengths from external connection electrodes to pixel portions of a display screen are not uniform. In the case of simple matrix driving with a duty of 1/240, a difference in line resistance of 500 .OMEGA. to 1 k.OMEGA. causes display nonuniformity. Therefore, it is difficult to attain high-density pattern layout. In the case of COG mounting of two or more chips, display nonuniformity or display defects may occur due to a high ITO resistance when a bus line is formed by utilizing panel electrodes. To solve this problem, auxiliary electrodes need to be formed by electroless plating, which will increase the cost. If the ITO electrodes are designed to be wide, the external size becomes large and the number of liquid crystal panels taken is reduced, resulting in a cost increase.
In the COG mounting of a liquid crystal electro-optical device, the solder bump connection causes a cost increase, because ITO is subjected to nickel electroless plating. Since the pitch cannot be reduced to less than 200 .mu.m, it is not suitable for high-density connection. In the case of the connection method using the anisotropic conductive film, a size variation of connecting particles need to be made about 5 .mu.m because bumps of an IC have a height variation of about 1 .mu.m, and the connecting particles need to be contained in an adhesive by more than 70 wt%. Thus, the cost becomes high. In conjunction with the above, since connection weighing of more than 50 g/bump is required, weight is applied to the IC in an unbalanced manner, reducing the yield. Further, since the anisotropic conductive film is temporarily attached to the substrate side in advance, it is difficult to perform pattern recognition in positioning the pads of the IC chip with respect to the ITO pattern, resulting in reduced productivity. Further, in the Ag paste method, the Ag transfer cannot be performed properly if the bump height of the IC chip is smaller than 30 .mu.m. Therefore, the bumps should be as high as 30-60 .mu.m, which causes a cost increase. Further, it is difficult to perform connection of a pitch less than 150 .mu.m, which causes limitations on the pattern layout of an IC chip. Since it is difficult to reduce the size of an IC chip, the cost is not decreased.
In a liquid crystal electro-optical device, since terminals of a glass substrate are connected to a film substrate by thermo-compression bonding, a positional deviation occurs between the terminals due to expansion of the film. Further, since the heating step requires the film to have heat resistance, stability of dimensions, etc., a polyimide film is generally used, which makes it difficult to attain a decrease in cost.